High hardness materials are widely used as coatings on various types of mechanical components and cutting tools. Such coatings impart erosive and abrasive wear resistance and thus increase the erosive and abrasive wear life of the objects that have been coated. High hardness materials can also be used to produce free standing objects which are erosive and abrasive wear resistant.
Chemical vapor deposition processes can be used to produce highly erosive and abrasive wear resistant hard coatings and free standing objects. In a typical chemical vapor deposition (CVD) process, the substrate to be coated is heated in a suitable chamber and then a gaseous reactant mixture is introduced to the chamber. The gaseous reactant mixture reacts at the surface of the substrate to form a coherent and adherent layer of the desired coating. By varying the gaseous reactant mixture and other CVD process parameters, various types of deposit coatings can be produced.
In co-pending U.S. patent application Ser. No. 92,809 filed 3 Sept. 1987, extremely hard, fine grained, non-columnar, substantially lamellar tungsten/carbon alloys are described which are produced by chemical vapor deposition. The described alloys consist primarily of a mixture of a substantially pure tungsten phase and at least one carbide phase wherein the carbide phase consists of W.sub.2 C or W.sub.3 C or a mixture of W.sub.2 C and W.sub.3 C. The disclosed tungsten/carbon alloys are free of columnar grains and consist essentially of extremely fine, equiaxial crystals. The deposits are produced at temperatures in the range of 300.degree. to 650.degree. C. At such low temperatures, degradation of the mechanical properties of the substrate is minimized as are problems resulting from wide differences between thermal coefficients of expansion in the substrate and the coating materials.
It has been found that the tungsten/carbon alloys such as those described in the aforementioned U.S. patent application, when deposited upon certain types of substrates, exhibit a very fine microcrack system throughout the deposit. On many types of substrates and under many types of erosive and abrasive wear conditions, preferential attack occurs at the cracks, resulting in reduced erosion and abrasion wear resistance of such coatings.
In co-pending U.S. patent application Ser. No. 07/153,738, filed 8 Feb. 1988, a highly erosive and abrasive wear resistant composite coating system is described in which an intermediate layer of substantially pure tungsten is deposited prior to depositing tungsten/carbon alloy coating. The outer tungsten/carbon alloy layer is comprised of a mixture of tungsten and tungsten carbide, with the tungsten carbide phase comprising of W.sub.2 C, W.sub.3 C or a mixture of both. The thickness of the intermediate tungsten layer is controlled in a manner to reduce or eliminate micro-cracks in the composite coating system and confer substantial erosive and abrasive wear characteristics on the composite coating system. The ratio of the thickness of the inner tungsten layer to the thickness of the outer tungsten/carbon alloy layer is at least above 0.3 in the cases of W+W.sub.3 C, W+W.sub.2 C+W.sub.3 C and W+W.sub.2 C coatings. Additionally, the ratio of the thickness of the inner layer to the thickness of the outer layer to get optimum erosion and abrasion wear performance is at least 0.35 in the case of mixtures of tungsten and W.sub.2 C in the outer layer, 0.60 in the case of mixtures of tungsten and W.sub.3 C in the outer layer and, 0.35 in the case of mixtures of tungsten and W.sub.2 C and W.sub.3 C in the outer layer.
The heat treating of various materials to improve properties is well known. A pertinent example of such heat treatment is described in detail in a paper by G. Hickey, et al. "Erosion of Conventional and Ultrafine-Grained Materials," Thin Solid Films, 118, pp. 321-333, 1984. In this article, the thermochemical deposition of a mixture of W--WC is described. This deposit, which is heavily microcracked, may be heat treated at a temperature of between 550.degree. and 600.degree. C. The coating is said to undergo a phase change at this temperature and results in a five fold increase in erosion resistance. See also D. G. Bhat and R. A. Holzl, --Microstructural Evaluation of CM 500L, A New W--C Alloy Coating Deposited by the Controlled Nucleation Thermochemical Deposition Process," Thin Solid Films, 95, pp. 105-112, 1982. Unfortunately, a phase change and presence of microcracks in some coatings may be undesirable. Moreover heat treatment at such temperatures may seriously degrade the mechanical properties of the substrate and may also result in the formation of undesirable brittle inter-metallic compounds at the boundary layer between the substrate and the protective interlayer.
The use of heat treatment below 550.degree. C. to enhance the microstructure and erosive and abrasive wear resistance properties of the coating without degrading the mechanical properties of the substrate is neither discussed nor presented in the prior art.